c++ - 为什么从输入收集字符串时整数输出为0

Question

我是 C++ 新手，正在研究 Stroustrup 的编程原则和实践。在第 3.3 节中，他给出了以下示例:

#include <iostream>
#include <string>

int main() {
  std::cout << "Please enter your first name and age\n";
  std::string firstName;
  int age;
  std::cin >> firstName >> age;
  std::cout << "Hello, " << firstName << " (age " << age << ")\n";
}

根据文本，我输入了“22 Carlos”，并期望输出为 22，后跟一些随机数或 0。很好，我得到 0。

然后我初始化变量并输入相同的“22 Carlos”:

string firstName = "???";
int age = -1;

我希望输出为“Hello, 22 (age -1)”，如文本中所述，但得到的却是“Hello, 22 (age 0)”。为什么？

Answer 1

您将age 设置为0 的原因是because (假设您使用的是 c++11 或更高版本):

If extraction fails (e.g. if a letter was entered where a digit is expected), value is left unmodified and failbit is set. (until C++11)

If extraction fails, zero is written to value and failbit is set. (since C++11)

顺便说明一下，VC++ 编译器直到最近才遵循此标准 https://developercommunity.visualstudio.com/content/problem/285201/the-value-is-not-zeroed-after-failure-but-it-shoul.html